What Is a Cron Job?
Cron is a time-based job scheduler built into Unix-like operating systems. A cron job is a command or script that runs automatically on a defined schedule. Cron reads configuration files called crontabs (cron tables) and executes the listed commands at the specified times.
Common uses: database backups, sending scheduled emails, clearing cache, rotating logs, running data sync scripts, and any recurring task you don't want to trigger manually.
Cron Syntax: The Five Fields
Every cron expression has exactly five time fields followed by the command to run:
┌───────────── minute (0–59)
│ ┌───────────── hour (0–23)
│ │ ┌───────────── day of month (1–31)
│ │ │ ┌───────────── month (1–12)
│ │ │ │ ┌───────────── day of week (0–6, Sunday=0)
│ │ │ │ │
* * * * * command to executeSpecial Characters
| Character | Meaning | Example |
|---|---|---|
* | Every value (wildcard) | * * * * * — every minute |
, | List of values | 0 9,17 * * * — at 9:00 and 17:00 |
- | Range of values | 0 9-17 * * * — every hour 9:00–17:00 |
/ | Step values | */15 * * * * — every 15 minutes |
L | Last (some systems) | 0 0 L * * — last day of month |
20 Practical Cron Examples
| Expression | When it runs |
|---|---|
* * * * * | Every minute |
*/5 * * * * | Every 5 minutes |
*/15 * * * * | Every 15 minutes |
0 * * * * | Every hour (at :00) |
0 */2 * * * | Every 2 hours |
0 0 * * * | Daily at midnight |
0 6 * * * | Daily at 6:00 AM |
0 9,17 * * * | Twice daily — 9 AM and 5 PM |
0 9-17 * * 1-5 | Every hour, 9–17, weekdays only |
0 0 * * 1 | Every Monday at midnight |
0 0 1 * * | 1st of every month at midnight |
0 0 1 1 * | January 1st at midnight (yearly) |
30 4 * * 0 | Every Sunday at 4:30 AM |
0 0 * * 1-5 | Midnight, Monday to Friday |
0 8 1-7 * 1 | First Monday of every month at 8 AM |
*/30 8-18 * * 1-5 | Every 30 min, 8–18, weekdays |
0 2 * * 6,0 | Saturdays and Sundays at 2 AM |
59 23 * * 5 | Every Friday at 23:59 |
0 12 15 * * | 15th of every month at noon |
5 4 * * sun | Sunday at 04:05 (using name) |
Predefined Schedule Strings
Most modern cron implementations support shorthand strings instead of the five-field syntax:
@reboot Run once at startup
@yearly Run once a year (= 0 0 1 1 *)
@annually Same as @yearly
@monthly Run once a month (= 0 0 1 * *)
@weekly Run once a week (= 0 0 * * 0)
@daily Run once a day (= 0 0 * * *)
@midnight Same as @daily
@hourly Run once an hour (= 0 * * * *)Working with Crontab
# Open your user crontab for editing
crontab -e
# List your current crontab
crontab -l
# Remove your crontab (careful — no confirmation!)
crontab -r
# Edit another user's crontab (as root)
crontab -u username -eOutput and Logging
By default, cron sends output as email to the local user. In practice, redirect output to a log file:
# Redirect stdout and stderr to a log file
*/5 * * * * /path/to/script.sh >> /var/log/myjob.log 2>&1
# Suppress all output (silent cron job)
*/5 * * * * /path/to/script.sh > /dev/null 2>&1
# Append to log with timestamps (using bash)
*/5 * * * * echo "$(date): job ran" >> /var/log/myjob.logPATH only includes /usr/bin:/bin. Always use full paths for commands and scripts, or set PATH at the top of your crontab.
Timezone in Cron
Cron uses the system timezone by default. To use a different timezone, set the CRON_TZ variable at the top of your crontab:
CRON_TZ=Europe/Istanbul
0 9 * * * /path/to/morning-report.shOn systems using systemd timers or newer cron implementations (Vixie cron, fcron), check /etc/crontab and /etc/cron.d/ for system-wide jobs.
Frequently Asked Questions
Why isn't my cron job running?
The most common causes: (1) wrong path — cron's PATH is minimal, always use full paths; (2) permissions — the script must be executable (chmod +x script.sh); (3) syntax error in the expression — test with an online validator first; (4) cron daemon not running (systemctl status cron); (5) output is going to mail and failing silently.
Can I run a cron job every 10 seconds?
No — the minimum interval for standard cron is 1 minute. For sub-minute scheduling, use sleep tricks (* * * * * sleep 30; command), systemd timers with OnCalendar=*:*:0/30, or a dedicated tool like fcron.
What is the difference between crontab and /etc/cron.d?
User crontabs (crontab -e) are per-user and have no username field. System crontabs in /etc/cron.d/ are run as root by default and include an extra username field between the time expression and the command: */5 * * * * www-data /usr/bin/php /var/www/cron.php.
How do I test a cron expression without waiting?
Use an online cron expression tester to see the next N run times, or temporarily set the job to run every minute and watch the logs. You can also test the command directly in your shell first, then add it to cron once confirmed working.